By observing polymerization reactions in cloud droplets a vast number of oxidation products have been identified even from a single precursor. In organic/inorganic mixed aerosol particles the presence of organic compounds significantly increases the number of cloud condensation nuclei by adding substantial mass to existing nucleation mode particles. Decadal trends in measured extinction coefficients revealed enhancement in visibility due to the decreasing concentrations of deliquescent inorganic particles. Atomic force microscopic studies implied that humic-like substances are present in the form of refractory films on particle surfaces. By applying cloud chemistry model for simulating multiphase reactions in cloud droplets it has been established that secondary organic aerosol formation can be globally significant in spite of its much lower yield relative to that of sulfate. Organic sulfate-esters have been shown to be ubiquitous and abundant in summer tropospheric aerosol and are likely formed in heterogeneous reactions. Measurements of heavy metals in aerosol and precipitation in the Balaton region have revealed that wet deposition is the major sink for these elements.